A known brake hydraulic pressure control apparatus controlling a hydraulic pressure of a wheel cylinder by means of a linear solenoid valve is disclosed in JP2005-162176A (which will be hereinafter referred to as Reference 1). The brake hydraulic pressure control apparatus disclosed in Reference 1 includes a master cylinder pressurizing a brake fluid within a reservoir by a brake pedal operation, cut valves arranged between the master cylinder and wheel cylinders, and a hydraulic pressure supply source including a hydraulic pump for applying a brake hydraulic pressure to the wheel cylinders. The brake hydraulic pressure control apparatus further includes pressure-increasing solenoid valves each formed by a linear solenoid valve and arranged between the hydraulic pressure supply source and the respective wheel cylinders. The wheel cylinders are connected to the reservoir via pressure-decreasing solenoid valves each formed by a linear solenoid valve.
In the brake hydraulic pressure control apparatus disclosed in Reference 1, when the brake pedal is operated in a state where the hydraulic pressure supply source is normally operated, the cut valves disconnect the master cylinder and the wheel cylinders from each other. The hydraulic pressure generated by the hydraulic pressure supply source is regulated and increased to a predetermined value by the pressure-increasing solenoid valves, and is applied to the wheel cylinders. In addition, the brake fluid at each of the wheel cylinders is discharged to the reservoir by operations of the pressure-decreasing solenoid valves so as to control the brake hydraulic pressure within the wheel cylinders in response to the operation of the brake pedal. The brake hydraulic pressure control apparatus disclosed in Reference 1 may include a buffer between the hydraulic pump and the reservoir so as to restrain an extreme decrease of an internal pressure of each of the pressure-decreasing solenoid valves caused by the pump operation. Such brake hydraulic pressure control apparatus is disclosed in JP2008-247354A (which will be hereinafter referred to as Reference 2).
According to the brake hydraulic pressure control apparatus disclosed in Reference 1, relief passages are connected to the respective pressure-decreasing solenoid valves. End portions of the relief passages are connected to a single circulating passage within a housing unit. The circulating passage is connected to the reservoir. The brake hydraulic pressure within each of the wheel cylinders is controlled or adjusted by the pressure-increasing solenoid valves and the pressure-decreasing solenoid valves formed by the linear solenoid valves. Such control is obtained by an adjustment of a valve opening of each of the solenoid valves that is achieved by means of a load balance between an electromagnetic force of a solenoid acting on a valve element, a pressure difference of the brake fluid, and a biasing force of biasing means. Specifically, according to each of the pressure-decreasing solenoid valves, because the valve element is brought to separate from a valve seat, the aforementioned load balance tends to deteriorate and an oscillation of the valve element (i.e., a self-oscillation) may occur. The self-oscillation that occurs at one of the pressure-decreasing solenoid valves causes pulsation of pressure at the relief passage to which the aforementioned pressure-decreasing solenoid valve is connected. The pressure pulsation that occurs at the aforementioned relief passage is transmitted to the other pressure-decreasing solenoid valves formed at the other relief passages. Accordingly, the load balance of the valve element in the other pressure-decreasing solenoid valve that receives the pressure pulsation may be deteriorated and the self-oscillation may occur, which leads to a cause of an operation noise (abnormal noise) of the brake hydraulic pressure control apparatus.
In addition, aeration in which air dissolved in the brake fluid turns to air bubbles in a valve chamber where the valve element is accommodated causes the self-oscillation of the valve element because of insufficient attenuation of the oscillation of the valve element. In such case, the self-oscillation generated at one of the pressure-decreasing solenoid valves is transmitted to the other pressure-decreasing solenoid valve(s), which leads to a cause of the operation noise of the brake hydraulic pressure control apparatus.
According to the brake hydraulic pressure control apparatus disclosed in Reference 1 where the cut valves disconnect the master cylinder from the wheel cylinders and the brake hydraulic pressure applied to the wheel cylinders from the hydraulic pump is controlled by the pressure-increasing solenoid valves and the pressure-decreasing solenoid valves formed by the linier solenoid valves, it is necessary to control the brake hydraulic pressure within each of the wheel cylinders without a generation of the operation noise upon brake operation under a normal driving condition of the vehicle. Thus, prevention of the operation noise upon operation of the brake hydraulic pressure control apparatus is an important issue in regards to the function of the brake hydraulic pressure control apparatus.
In order to decrease an occurrence of self-oscillation of the pressure-decreasing solenoid valve, the buffer is provided between the hydraulic pump and the reservoir so that the extreme decrease of the internal pressure of the pressure-decreasing solenoid valve caused by the pump operation is restrained as disclosed in Reference 2. However, the resulting structure is complicated, which leads to an increase of size and cost of the brake hydraulic pressure control apparatus.
A need thus exists for a brake hydraulic pressure control apparatus which is not susceptible to the drawback mentioned above.